Primary active transporters ATPases

Besides by interfering with the general energy maintenance of the cell, impact of active transport during intracellular accumulation can also be investigated by direct inhibition of certain transport mechanisms. To detect contribution of Na+-dependent transport, the Na+/K+-ATPase, one of the primary active transport systems of the cell, can be inhibited by ouabain at 10 /uM. Higher amounts of the inhibitor may seriously impede with the viability of the cell. To obtain reliable results, the cells are pretreated with ouabain for 15 min prior to addition of the analyte. Usually, the inhibitory effect of ouabain lasts up to 45 min [29],... 

In virtually every animal cell type, the concentration of Na+ is lower in the cell than in the surrounding medium, and the concentration of K+ is higher (Fig. 11-36). This imbalance is maintained by a primary active transport system in the plasma membrane. The enzyme Na+K+ ATPase, discovered by Jens Slcou in 1957, couples breakdown of ATP to the simultaneous movement of both Na+ and K+ against their electrochemical gradients. For each molecule of ATP converted to ADP and I , the transporter moves two K+ ions inward and three Na+ ions outward across the plasma membrane. The Na+K+ ATPase is an integral protein with two subunits (Mr -50,000 and -110,000), both of which span the membrane. 

Direct coupling of adenosine triphosphate (ATP) hydrolysis is an example of an active transport process. The most important of these in the nephron is Nafi K -ATPase, which is located on the basolateral membranes of the tubulo-epithelial cells (Figure 45-6). This enzymatic transporter accounts for much of renal oxygen consumption and drives more than 99% of renal sodium reabsorption. Other examples of primary active transport mechanisms are a Ca " -ATPase, an H -ATPase, and an H, K -ATPase. These enzymes establish ionic gradients, polarizing cell membranes and thus driving secondary transport processes. 

The Na -K ATPase Pump. The Na -K" " adenosine triphosphatase (ATPase) transport system is the primary active transport process that maintains the normal chemical gradients for Na" " and K" " in cells of both vertebrates and invertebrates. Although the concern of this article is with neuronal tissue, it should be stressed that the Na -K ATPase pump... 

The H, K-ATPase protein complex is also a primary active transporter, which moves hydrogen ions out of the cell and potassium ions into the cell against their electrochemi-... 

Primary Active Transport Membrane transport that directly couples with ATP hydrolysis is called primary active transport. ABC transporters are examples of primary active transporters. They contain one or two highly conserved ATP binding cassettes that exhibit ATPase activity. ABC transporters mediate the unidirectional efflux of many solutes across biological membranes. 

Primary active transport systems that have been studied include the sodium-b potassium pump [25,26] (which is the sodium + potassium-activated ATPase) and the calcium ATPase [27] and include also the ATP synthesising proton pumps of mitochondria [18,28]. 

On the other hand, there are two groups or systems for various nutrients and metabolites in which transport of certain metabolites postulated to be driven directly by the hydrolysis of ATP or other energy-rich compounds without the requirement of a protonmotive force these are the binder -requiring systems and the phosphotransferase systems [22]. Binders are periplasmic proteins, which occur between the cytoplasmic and the outer membrane of microorganisms. They appear to be loosely attached to the outer face of the cytoplasmic membrane, from which they can be removed, for instance, by mild osmotic shock. However, the mentioned hypothesis that the (shock-sensitive) binder systems of active transport are primary active does not appear to be fully established mainly because a distinction between primary and secondary active transport is not possible in the presence of a functioning H -transporting ATPase. The main evidence in favor of primary active transport in shock-sensitive transport systems has been derived from a Ca, Mg, ATPase-less mutant of E. coli. There are, however, some findings which do not seem to support this hypothesis. 

After Chapter 1 on non-mediated transport of lipophilic compounds. Chapters 2 and 3 are devoted to the passive transport of water and other small polar molecules and to that of ions. Chapter 4 discusses the insertion of ionophores in lipid bilayers as model systems for carriers and channels in biological membranes. Chapter 5 treats the general principles of mediated transport. Chapters 6, 7 and 8 are devoted to the ATPases, which are involved in the primary active transport of Na, Ca and H, respectively. After Chapters 9 and 10 on specific transport systems in mitochondria and bacteria, the book concludes with Chapters 11 and 12 on secondary active transport, the coupling of the transport of metabolites and water to that of ions. 

Transporters mediate the admission or delivery of lipid-unsoluble materials (sugars, amino acids, ions) via the phospholipid membranes of the cell. Transporters do not have a porous structure, Wt transport their freight in three steps binding of the substrate to one side of the membrane, conformation change or position change of the transporter-substrate complex, and dissociation of the substrate from the transporter on the other side of the membrane. Often, not only one molecule is being transported but several at the same time, either in the same direction (symport e.g., and lactose) or in opposite direction (antiport e.g., Na versus Ca " ). Primary active transport processes (pumps) are powered by ATPases (e.g., Na" /... 

Primary active transport occurs when the transport of a substrate is coupled to an energy-yielding metabolic reaction. The energy required may come from several different sources (a) the high-energy compound ATP used by a specific ATPase (ATPase pump) (b) energy from the electron transport system released as electrons that flow down the cytochrome chain (redox-pump) and (c) the electric field produced by free radicals. Implicit in these three theories is the participation of ions and ion transport. Secondary active transport is a term often used to denote the transport of one substrate linked to the flow of a second substrate. Wilbrandt (1975) refers to this as flow-coupled active transport it may be this form of transport that is most often involved in the active uptake of sugars and amino acids. A review of some models of carrier-mediated active transport transport has recently been presented by Crane (1977). 

K+-ATPase is the primary source of the membrane potential for most eukaryotic cells and is said to be electrogenic. Because the cell membrane is somewhat permeable to K+, outward diffusion of K+ through the "leaky" membrane along its concentration gradient helps to maintain the membrane potential as does inward leakage of CP. At the same time, Na+ diffuses inward, aided by the membrane potential. Even though the permeability of Na+ is low, a steady state is reached at which the rate of passive inward diffusion of cations just balances the membrane potential set up by the active transport. 

In addition to the Na+,K+- ATPases there is a very active Ca2+-ATPase which transports two Ca2+ from the inside of cells to the outside while returning two H+ from outside per ATP.510 543a This is the primary transporter by which cells maintain a low internal [Ca2+]. During its action it becomes phosphorylated on Asp 351. However, in neurons, in which the membrane potential is maintained at a high negative value by the sodium pump, an Na+/ Ca2+ ion exchange plays an even more important role.540... 

Figure 9.29 Some mammalian (left) and microbial (right) membrane transport systems. (A) Primary electrogenic mechanisms (pumps) creating either a Na+ or a H+ gradient. (B) Secondary active transport systems of the symport type, in which the entry of a nutrient S into the cell is coupled with the entry of either the sodium ions or protons. (D) Various passive ion movements, possibly via channels or uniports. (Reproduced by permission from Serrano R. Plasma Membrane ATPase of Plants and Fungi. Boca Raton CRC Press, 1985, p. 59.)...

In addition to the Na, K - ATPases there is a very active Ca " -ATPase which transports two Ca + from the inside of cells to the outside while returning two H from outside per This is the primary... 

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